Effing Marriage...

[Fitz]

I'm not married yet, though will eventually marry my girl. But we have 2 kids together, so figured to post here anyways.
We don't fight, always communicate and we have never had a full blown fight, though one issue that has always popped up (even though we haven't fought about it), and we have a civil and thought out debate about is smacking/spanking our children.
She is against it, and I am for it. For the record, I have only smacked my oldest child, who is 3 and have only done it 4-5 times. Also my form of smacking have only consisted of a slap across his hands, and a firm whack across his backside.

She says everything she has read says it can have a long term negative effect on the child. I tell her that she reads material on her parenting methods and of course what she reads will be bias towards her beliefs, just as if I were to read material on my beliefs that would be bias to my beliefs.
She is really good about it, and we both see where each other is coming from. She just doesn't think it is natural for me to smack my kid because I always feel bad afterwards. I tell her that most parents wouldn't feel great punishing their child and it would be more of a concern if I got a rush over it. I don't like to do it, but in some circumstances I think I need to (not because I want to).
For example last night, my oldest one was getting tired, emotional and a little angry. My youngest one was standing in the lounge room facing me. The oldest one ran into the lounge room having a bit of a meltdown and pushed him from behind for absolutely no reason and the little one smacked his head hard onto the ground.
I grabbed him straight away, gave him a whack across the ass, dragged him into his room and told him that his behaviour was disgusting and put him into his room. My girl was in the kitchen at the time, but I told her what I did, and she was upset about it later, but 'kind of understood' why I reacted like that, but didn't agree.
Does anyone have conflicting views to disciplining their children with their partner?